In my opinion, X-Men: First Class doesn't get enough credit as an absolutely stellar prequel. The franchise looked dead in the water after X-Men 3 and X-Men Origins: Wolverine. X-Men: First Class not only revitalized the franchise, but brought a sense of fun and color that the series had been devoid of previously. While Sir Patrick Stewart and Sir Ian McKellen cannot be beat; James McAvoy and Michael Fassbender do phenomenal jobs with the roles, themselves.

I know many see X2 as the highpoint of the series (or maybe Days of Future Past), but for me it would likely be X-Men: First Class